1. Field of the Invention
The present invention relates generally to gas chromatography and more particularly to a thermal conductivity detection device wherein a change in sample gas stream composition is identified pursuant to the measurement of electrical variations in electrical circuitry.
2. Description of the Prior Art
In detecting changes in gas stream composition, a detection device sensitive to the thermal conductivity of the sample gas is normally used. The sensitive component of the detector is a resistance filament located within a sample chamber. An electrical current is passed through the filament resulting in a filament equilibrium temperature above that of the chamber wall and that filament temperature is determined by the thermal balance of the electrical heat input to the filament minus the thermal losses of the filament. Design considerations permit the primary thermal loss of the filament to be by thermal conduction of heat through the surrounding gas stream within the sample chamber. Because the thermal conductivity of a gas stream is dependent upon the chemical composition of the gas, the equilibrium temperature of the filament is also dependent upon the composition of the gas. Therefore, thermal conductivity detectors identify changes in gas composition by sensing temperature changes in the resistance filament.
The thermal conductivity theory can be realized because a unique relationship exists between the temperature of the resistance filament and the electrical resistance of the resistance filament. The filament electrical resistance is sensed by including this filament as one component of a balanced resistance network known as a Wheatstone bridge. An electrical current is supplied to the resistance components of the bridge which causes a reduction in potential difference across and an increase in temperature of each resistance component. Any resistance imbalance of the resistance filaments can be determined by detecting any voltage imbalance measured at the bridge network midpoint.
Normally, two resistance filaments are exposed to a mixture of sample gas and a reference gas of fixed composition and two resistance filaments are exposed only to the reference gas. The reference gas usually has a higher thermal conductivity than the mixture and as the concentration of the sample gas increases, the thermal conductivity of the mixture decreases causing the temperature and electrical resistance of the exposed resistance filaments to increase. The resulting voltage imbalance of the Wheatstone bridge caused by the increase in resistance of the filament represents the difference in thermal conductivity between the sample gas and the reference gas. The thermal conductivity detector's sensitivity to differences in thermal conductivity increases with increasing current supplied to the bridge.
Four common methods are used to power the detector. One method applies a fixed voltage across the detector and a second method applies a constant current to the detector. In both methods, the detector output signal is the voltage imbalance at the bridge midpoint. A third method is the constant temperature method which maintains a balanced bridge by use of feedback voltage control which is the output signal. The fourth method is a constant mean temperature technique which utilizes a bridge circuit within a second bridge circuit and feedback control. Its output signal is the voltage imbalance across the inner bridge. All methods of powering the detector produce response characteristics which are output signals linearly proportional to the sample concentration only at low sample concentrations. The non-linear response of the thermal conductivity detector exists because the theoretical relationship between the detector output voltage and sample concentration is not a linear relationship. In order for the detector to be an ideal linear device, the following relationship must exist. The detector output voltage must be a linear function of the change in resistance of the resistance filaments exposed to the sample gas, which exist for the constant current method of operation but not for the constant voltage method. Also, the change in resistance of the resistance exposed to the sample gas must be a linear function of the thermal conductivity of the gas mixture in the sample cavity. In general, this relationship is not linear and only becomes approximately linear when the change in resistance of the resistance exposed to the sample gas is very small compared to the resistance exposed to the reference gas and the number of sample gas molecules is negligible compared to the number of reference gas molecules. Finally, the thermal conductivity of the gas mixture must be a linear function of the sample concentration and again, this relationship is generally not linear. Again. only for a small number of sample molecules compared to the number of reference gas molecules, does this relationship reduce to an approximate linear relationship. Thus, at sample concentrations above ten percent, the output signal is a non-linear result of sample concentration causing a loss in detector sensitivity.
Prior solutions to this non-linearity problem have been limited to the use of calibration curves for each sample tested, otherwise, the thermal conductivity detector method could not be used for high sample concentrations.